In microscopes, which along with an eye piece, also have a photographic camera for the recording of the observed image of an object, it is known that sharp focusing of the image cast onto the image-recording plane of the camera can be obtained by use of a focusing reticle. In so doing, the image of the focusing reticle and of the object is sharply adjusted in the image plane of the eyepiece.
We can distinguish three basic methods for precise focusing. As the first of these, we can mention the direct observation of the image of the object occuring on a focusing screen of the camera (image recording device) by means of a loupe. This is the simplest but also the least economical method, because it requires adjustments to the camera, and such adjustments are time-consuming.
Greater convenience is afforded by means of a separate special eyepiece having a part of the light conducted to the camera and part diverted to the special photographic eyepiece. However, this requires an expensive construction, and moreover is unwieldy in use because the observer has to shift from the normal observation eyepiece position to a different position for using the precision focusing eyepiece. Continuous observation of the object is necessarily interrupted when this apparatus is used.
It is known also to directly introduce the precision focusing system into the observation eyepiece. The image of a focusing reticle is superimposed over the image of the object being observed and the focusing reticle can be arranged in one or both eye pieces of a binocular tube. The latter arrangement is certainly better than those described above, but does present disadvantages. Thus, when using the microscope for purely visual observation, either the focusing reticle and the eye piece must be exchanged, or else its distraction caused by superimposition on the image must be tolerated. Furthermore, it is disadvantageous for the focusing reticle to be seen only by one eye, because this impairs precise focusing. With optionally removeable focusing reticle, the reproducibility of precise positioning of the reticle is not fully guaranteed.
Another known arrangement utilizes an intermediate image of the object, which together with the image of a focusing reticle, is viewed in an observational device made up in the form of a binocular tube. In this arrangement there is located, between a beam-splitting prism, which divides the beam path issuing from the object into one light path leading to the camera, and one light path leading to the observational device, and to the intermediate plane in the eye piece of the observational light path, a further intermediate image plane, in which the focusing reticle is located. Here, certainly, an observation of the focusing reticle with both eyes is possible; however, such types of arrangements by their very nature are very involved and expensive and/or they detract from the quality of the visual image possible because of the number of optical elements required.
In addition to all these disadvantages, a further disadvantage is inherent in all known devices. The resolution of the focusing reticle is low because it is presented as being dark on a light background, whereby the narrow dark lines on dark objects are difficult to see. This is especially true where dark field illumination and fluorescence microscopy techniques are used.